The present invention relates to a capacitor and a method for fabricating the same and a semiconductor device, and a method for fabricating the same, more specifically to a ferroelectric capacitor having a ferroelectric film having an electric field application direction and a polarization axis which are parallel with each other, a method for fabricating the same and a semiconductor device having such the ferroelectric capacitor and a method for fabricating the same.
Ferroelectric materials, such as SrTiO3, Pb(Zr,Ti)O3, etc., have applications in various fields where their high dielectric constants and polarization inversion characteristics are utilized. An example of the applications utilizing their high dielectric constants is DRAM-type semiconductor memory devices comprising capacitors including a ferroelectric film as the dielectric film (ferroelectric capacitors), for storing informations in the capacitors as electric charges. An example of the applications utilizing polarization inversion characteristics is nonvolatile memory devices comprising ferroelectric capacitors, for storing in the capacitors informations corresponding to polarization directions of the ferroelectric film. Ferroelectric capacitors can have capacitor areas decreased by increasing capacitance values per unit area. Ferroelectric capacitors can form nonvolatile devices. Ferroelectric capacitors are very useful in semiconductor memory devices which are increasing micronized.
A conventional capacitor using ferroelectric film will be explained with reference to FIGS. 19A–19C. FIGS. 19A–19C are diagrammatic sectional views of the conventional capacitors.
As exemplified in FIG. 19A, the conventional capacitor comprises a lower electrode 100 of, e.g., platinum, a ferroelectric film 102 of, e.g., Pb(Zr,Ti)O3 (hereinafter called PZT), and an upper electrode 104 of, e.g., platinum which are laid on another.
Usually, the platinum film as the lower electrode 100 is polycrystal and strongly (111) oriented (see, e.g., Journal of Applied Physics, 1991, vol. 70, No. 1, pp. 382–388). In this case, when the ferroelectric film 102 is formed of PZT having a Zr/Ti composition ratio of below 0.52/0.48 and tetragonal system crystal structure, the PZT film is strongly also (111) oriented under the influence of the platinum film whose lattice structure is similar.
In applying such ferroelectric capacitors to an nonvolatile memory device, information is written by controlling polarization directions of the ferroelectric film. Polarization directions of PZT having tetragonal system are <001> direction because average positions of plus ions and minus ions are offset from each other in <001> direction. Accordingly, in the ferroelectric capacitor having the (111) oriented PZT film, as shown in FIG. 19B, polarization directions (indicated by the arrows in the drawing) of the PZT film are oblique to a voltage application direction. Consequently, with respect to a voltage application direction of the capacitor, the polarization which can be obtained is smaller than an intrinsic polarization of PZT.
A region where directions of polarization are aligned is called a domain. In (111) oriented PZT, as shown in FIG. 19B, a domain wall (180° domain wall 106) across which domains having polarization directions different from each other by 180° are adjacent to each other, and a domain wall (90° domain wall 108) across which domains having polarization directions different from each other by 90° are adjacent to each other are present. Upon the polarization inversion at the time of application of a voltage, no stress takes place in the 180° domain wall 106, but stresses take place in the 90° domain wall 108. Characteristics of the ferroelectric capacitor, data retention characteristics especially in nonvolatile memory devices are much degraded. Accordingly, to fabricate an nonvolatile memory device having good characteristics it is preferable that a ferroelectric film having no 90° domain wall 108 but having only the 180° domain wall 106 is used.
Ferroelectric films having 180° domain walls alone are, e.g., (001) oriented tetragonal PZT film and (111) oriented rhombohedral PZT film. As shown in FIG. 19C, the (001) oriented tetragonal PZT film and the (111) oriented rhombohedral PZT film have no 90° domain wall and has 180° domain walls alone. Furthermore, a voltage application direction and polarization directions (indicated by the arrows in the drawing) of the capacitor are parallel with each other, whereby the intrinsic polarization intensity of the substance can be utilized as it is in the ferroelectric capacitor.
To form (001) oriented PZT film, single crystal (100) MgO substrate and single crystal (100) SrTiO3 substrate have been used as substrates. As shown in FIG. 20, platinum film is deposited on, e.g., a single crystal (100) MgO substrate by sputtering method at high temperature, whereby (100) oriented platinum film 112 can be formed on the MgO substrate 110 under the influence of the planar orientation of the MgO substrate 110. PZT film is deposited on the (100) oriented platinum film 112, whereby a (001) oriented PZT film 114 can be formed under the influence of the orientation direction of the platinum film (see, e.g., Journal of Applied Physics, 1991, vol. 69, No. 12, pp. 8352–8357).
FIG. 21 is a graph of data retention characteristics of an nonvolatile memory device using a ferroelectric capacitors including (111) oriented PZT film, and an nonvolatile memory device using ferroelectric capacitors including (001) oriented PZT film. The ferroelectric capacitor including the (111) oriented PZT film comprises a lower electrode of (111) oriented platinum film formed on a silicon substrate with a silicon oxide film formed on, and the (111) oriented PZT film formed on the (111) oriented platinum film. The ferroelectric capacitor including the (001) oriented PZT film comprises a lower electrode of the (100) oriented platinum film formed on a (100) MgO substrate, and the (001) oriented PZT film formed on the (100) oriented platinum film. In the graph, retention times after data writing are taken on the horizontal axis, and normalized polarization is taken on the vertical axis.
As shown, in the case that the (111) oriented PZT film is used, the polarization decrease as the retention times increase, while in the case that the (001) oriented PZT film is used, decreases of polarization can be suppressed.
In the nonvolatile semiconductor memory device using ferroelectric capacitors, the ferroelectric capacitors are formed over a silicon substrate with active elements formed on, interposing an amorphous insulation film therebetween. Platinum film as the lower electrodes is formed on an adhesion layer of, e.g., TiO3 film on the amorphous insulation film. The thus-formed platinum film becomes (111) oriented film. Thus in the conventional nonvolatile memory device, the PZT film formed on the platinum film also becomes (111) oriented film. Ferroelectric capacitors including (001) oriented PZT having good data retention characteristics cannot be formed.
A method of forming (100) oriented platinum film on an amorphous insulation film on a silicon substrate by sputtering method using Ar gas and O2 gas is described in, e.g., Journal of Material Research, 1999, vol. 14, No. 3, pp. 634–637. PZT deposited on the (100) oriented platinum film becomes (100) oriented PZT film, and (001) oriented PZT film cannot be formed. (100) oriented PZT film has the polarization direction which is perpendicular to an electric filed application direction, and the resultant polarization is very small.
As described above, in the conventional capacitors using the ferroelectric materials, especially formed over a silicon substrate interposing an amorphous insulation film therebetween, PZT film having a polarization axis parallel with an electric field application direction cannot be formed. Nonvolatile memory devices using such capacitors could not have sufficient data retention characteristics.